The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink, while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
The type of lithographic printing plate to which the present invention is directed has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when that portion of the coating which is exposed becomes hardened, the plate is referred to as negative-working. In both instances the image area remaining is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then comes more soluble and is removed. In the case of a negative plate, the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
One of the more serious problems which can afflict negative-working lithographic printing plates is inability of the developer to remove all residual coating from the non-image areas of the plate. When sufficient residual coating remains, a condition exists for background sensitivity to occur during the printing process. Minimally, the effect would be to increase the amount of water required in the fountain solution. Under more severe conditions, ink may adhere to the background and ultimately to the printed sheet, thereby resulting in a condition known as "tinting" or "toning." Under extremely severe conditions, there is so much ink in the background that it is referred to as "scumming."
In the manufacture of lithographic printing plates having an aluminum support, it is well known to provide a hydrophilic barrier layer interposed between the aluminum support and a radiation-sensitive layer which forms the lithographic printing surface. The barrier layer is utilized primarily to improve the hydrophilicity of the background areas and thereby minimize such problems as scumming. Many different materials have been proposed for use in forming such barrier layers. The barrier layer can be applied directly to the surface of the aluminum sheet material or the aluminum can be grained and/or anodized prior to application of the barrier layer. Illustrative of the many materials useful in forming such barrier layers are polyvinyl phosphonic acid, polyacrytic acid, polyacrylamide, silicates, zirconates and titanares. Included among the many patents relating to hydrophilic barrier layers utilized in lithographic printing plates are U.S. Pat. Nos. 2,714,066, 3,181,461, 3,220,832, 3,265,504, 3,276,868, 3,549,365, 3,902,976, 4,090,880, 4,153,461, 4,376,814, 4,383,987, 4,399,021, 4,427,765, 4,427,766, 4,448,647, 4,452,674, 4,458,005, 4,492,616, 4,578,156, 4,689,272, 4,935,332 and European Patent No. 190 643.
Polymers containing phosphonic acid groups have been found to be especially beneficial in forming hydrophilic barrier layers, and use of such polymers has been described in at least the following references:
(1) Use of polyvinyl phosphonic acid, a copolymer of vinylphosphonic acid and acrylic acid or a copolymer of vinylphosphonic acid and vinyl acetate to treat aluminum which has not been anodized is proposed in U.S. Pat. No. 3,220,832. The process utilized is one which does not employ electric current.
(2) Use of polyvinyl phosphonic acid to treat anodized aluminum is proposed in U.S. Pat. No. 4,153,461. Again, the process utilized is one which does not employ electric current.
(3) Use of a mixture of polyvinyl phosphonic acid and sodium silicate to treat aluminum in either a thermal or electrochemical process is proposed in U.S. Pat. No. 4,376,814.
(4) Use of an electrolyte solution containing a water-soluble organic acid, such as polyvinyl phosphonic acid, and a phosphorus oxo acid to treat aluminum in an electrochemical process employing pulsed direct current is proposed in U.S. Pat. No. 4,383,897.
(5) Use of a water-soluble polybasic organic acid, such as polyvinyl phosphonic acid, to treat aluminum in an electrochemical process employing pulsed direct current is proposed in U.S. Pat. No. 4,399,021.
(6) Use of a reaction product of polyvinyl phosphonic acid and a salt of a divalent metal cation to treat aluminum, with or without an anodic coating of aluminum oxide, in a thermal process is proposed in U.S. Pat. No. 4,427,765.
(7) Use of an electrolyte solution containing a water-soluble organic acid, such as polyvinyl phosphonic acid, and a phosphorus oxo acid to treat aluminum in a direct current electrochemical process is proposed in U.S. Pat. No. 4,448,647.
(8) Use of a water-soluble polybasic organic acid, such as polyvinyl phosphonic acid, to treat aluminum in a direct current electrochemical process is proposed in U.S. Pat. No. 4,452,674.
(9) Use of polyvinyl methylphosphinic acid to treat anodized aluminum in a thermal process is proposed in U.S. Pat. No. 4,458,005.
(10) Use of a mixture of polyvinyl phosphonic acid and sufficient base to form an electrolyte with a pH of 3 to 10 to treat aluminum in an electrochemical process is proposed in U.S. Pat. No. 4,578,156.
(11) Use of polyvinyl phosphonic acid, a copolymer of vinylphosphonic acid and acrylic acid, a copolymer of vinylphosphonic acid and vinyl acetate or a copolymer of vinylphosphonic acid and acrylamide to treat anodized aluminum in a thermal process in which the aluminum oxide layer is treated with a silicate and then with a solution containing the polymer or copolymer is proposed in U.S. Pat. No. 4,689,272.
(12) Use of a homopolymer of acrylamide isobutylene phosphonic acid or a copolymer of acrylamide isobutylene phosphonic acid and acrylamide to treat aluminum, with or without an anodic coating of aluminum oxide, in a thermal or electrochemical process is proposed in South African Patent Specification No. 860,876.
While the hydrophilic barrier layers proposed heretofore are beneficial, there is a continuing need in the art for a more effective barrier layer which will render the treated surface both hydrophilic and stable and will provide excellent adhesion for subsequently applied negative-working or positive-working radiation-sensitive coatings.
Aluminum and aluminum oxide are neither oleophilic nor hydrophilic. They are referred to as amphipathic. Hence, the need for a barrier layer which will render the aluminum surface, and thus the background or non-image area of the printing plate, hydrophilic. The barrier layer not only provides a hydrophilic surface, but can also protect components of the radiation-sensitive layer, such as diazo resins, from undesirable interactions with the aluminum and can provide for improved adhesion of the radiation-sensitive layer. The barrier layer must provide a stable hydrophilic surface, since a major concern in the lithographic printing process is the ability of the background to remain clean by not being subject to oxidation or sensitization, and by not having ink-receptive material become part of the surface.
It is toward the objective of providing an improved barrier layer, that more effectively meets the needs of the lithographic printing industry, that the present invention is directed.